A main shaft of a turbo-machine (for example, a gas turbine or a turbocharger) is rotated at high speed under a high-temperature environment. Further, there is a case in which the turbo-machine has a difficulty in being provided with a separate auxiliary machine for oil circulation from the viewpoints of energy efficiency, and there is also a case in which shear resistance of a lubricating oil may be a factor for inhibiting high-speed rotation of the main shaft. Therefore, as a bearing for supporting the main shaft of the turbo-machine, an air dynamic pressure bearing using air as pressure-generating fluid is often used in place of a rolling bearing or a dynamic pressure bearing using oil lubrication.
In the air dynamic pressure bearing, both a bearing surface on a rotary side and a bearing surface on a stationary side are generally constructed of rigid bodies. However, in this type of air dynamic pressure bearing, when management of a gap width of a bearing gap formed between both the bearing surfaces is insufficient, self-excited whirling called a “whirl” becomes more liable to occur in the shaft upon exceeding a stability limit. Thus, in a general air dynamic pressure bearing, the gap width of the bearing gap needs to be managed with high accuracy in order to stably exhibit bearing performance. However, under an environment in which a temperature change is large as in the turbo-machine, the gap width of the bearing gap is easily varied under the influence of thermal expansion. Therefore, there is difficulty in stably exhibiting the bearing performance.
There has been known a foil bearing as a bearing which is less liable to cause the whirl and enables management of a gap width of a bearing gap to be performed easily even under the environment in which a temperature change is large. The foil bearing has a bearing surface which is constructed of a flexible metal thin plate (foil) having low rigidity against bending. The foil bearing allows flexure of the bearing surface, to thereby support a load. The foil bearing has a feature in that the bearing gap is automatically adjusted to an appropriate width in accordance with, for example, an operation condition. For example, in Patent Literature 1 described below, there is disclosed an example of a radial foil bearing configured to support a radial load.
Incidentally, in the foil bearing, particularly during low-speed rotation of the shaft, rigidity (pressure) of the air film formed in the bearing gap is not sufficiently increased, and hence the bearing surfaces repeatedly come into slide contact with each other. In order to suppress abrasion of the bearing surfaces and increase in rotational torque due to such slide contact, in Patent Literature 1, there is disclosed that a coating film such as a DLC film, a titanium aluminum nitride film, or a molybdenum disulfide film is formed on a surface of each foil forming the bearing gap with a shaft.